1. Field of the Invention
The present invention relates to a molten metal supply system and, more particularly, a continuous pressure molten metal supply system and method for forming continuous metal articles of indefinite length.
2. Description of the Prior Art
The metal working process known as extrusion involves pressing metal stock (ingot or billet) through a die opening having a predetermined configuration in order to form a shape having a longer length and a substantially constant cross-section. For example, in the extrusion of aluminum alloys, the aluminum stock is preheated to the proper extrusion temperature. The aluminum stock is then placed into a heated cylinder. The cylinder utilized in the extrusion process has a die opening at one end of the desired shape and a reciprocal piston or ram having approximately the same cross-sectional dimensions as the bore of the cylinder. This piston or ram moves against the aluminum stock to compress the aluminum stock. The opening in the die is the path of least resistance for the aluminum stock under pressure. The aluminum stock deforms and flows through the die opening to produce an extruded product having the same cross-sectional shape as the die opening.
Referring to FIG. 1, the foregoing described extrusion process is identified by reference numeral 10, and typically consists of several discreet and discontinuous operations including: melting 20, casting 30, homogenizing 40, optionally sawing 50, reheating 60, and finally, extrusion 70. The aluminum stock is cast at an elevated temperature and typically cooled to room temperature. Because the aluminum stock is cast, there is a certain amount of inhomogeneity in the structure and the aluminum stock is heated to homogenize the cast metal. Following the homogenization step, the aluminum stock is cooled to room temperature. After cooling, the homogenized aluminum stock is reheated in a furnace to an elevated temperature called the preheat temperature. Those skilled in the art will appreciate that the preheat temperature is generally the same for each billet that is to be extruded in a series of billets and is based on experience. After the aluminum stock has reached the preheat temperature, it is ready to be placed in an extrusion press and extruded.
All of the foregoing steps relate to practices that are well known to those skilled in the art of casting and extruding. Each of the foregoing steps is related to metallurgical control of the metal to be extruded. These steps are very cost intensive, with energy costs incurring each time the metal stock is reheated from room temperature. There are also in-process recovery costs associated with the need to trim the metal stock, labor costs associated with process inventory, and capital and operational costs for the extrusion equipment.
Attempts have been made in the prior art to design an extrusion apparatus that will operate directly with molten metal. U.S. Pat. No. 3,328,994 to Lindemann discloses one such example. The Lindemann patent discloses an apparatus for extruding metal through an extrusion nozzle to form a solid rod. The apparatus includes a container for containing a supply of molten metal and an extrusion die (i.e., extrusion nozzle) located at the outlet of the container. A conduit leads from a bottom opening of the container to the extrusion nozzle. A heated chamber is located in the conduit leading from the bottom opening of the container to the extrusion nozzle and is used to heat the molten metal passing to the extrusion nozzle. A cooling chamber surrounds the extrusion nozzle to cool and solidify the molten metal as it passes therethrough. The container is pressurized to force the molten metal contained in the container through the outlet conduit, heated chamber and ultimately, the extrusion nozzle.
U.S. Pat. No. 4,075,881 to Kreidler discloses a method and device for making rods, tubes, and profiled articles directly from molten metal by extrusion through use of a forming tool and die. The molten metal is charged into a receiving compartment of the device in successive batches that are cooled so as to be transformed into a thermal-plastic condition. The successive batches build up layer-by-layer to form a bar or other similar article.
U.S. Pat. Nos. 4,774,997 and 4,718,476, both to Eibe, disclose an apparatus and method for continuous extrusion casting of molten metal. In the apparatus disclosed by the Eibe patents, molten metal is contained in a pressure vessel that may be pressurized with air or an inert gas such as argon. When the pressure vessel is pressurized, the molten metal contained therein is forced through an extrusion die assembly. The extrusion die assembly includes a mold that is in fluid communication with a downstream sizing die. Spray nozzles are positioned to spray water on the outside of the mold to cool and solidify the molten metal passing therethrough. The cooled and solidified metal is then forced through the sizing die. Upon exiting the sizing die, the extruded metal in the form of a metal strip is passed between a pair of pinch rolls and further cooled before being wound on a coiler.
An object of the present invention is to provide a molten metal supply system that may be used to supply molten metal to downstream metal-working or forming processes at substantially constant working pressures and flow rates. It is a further object of the present invention to provide a molten metal supply system and method capable of forming continuous metal articles of indefinite lengths.